Gas Cooker Oven problem?

Hi, dunno if anyone can help but my cooker is on the blink.

Right, the problem is the oven. When it is switched on, it lights up but its as though its stuck on gas mark 1 and the flame never rises.

I've tried cleaning it as I thought originally there might just be a bit of gunk in there but no joy.

Does anyone have any idea's please, I'm sure its an easily solved problem, its just doing my head in!

Cheers